CSC 360, Instructor: Kui Wu Thread & PThread

CSC 360, Instructor: Kui Wu Agenda 1.What is thread? 2.User vs kernel threads 3.Thread models 4.Thread issues 5.Pthread library 6.An example

CSC 360, Instructor: Kui Wu 2 1. What is thread (1): Program, process, thread In one process easy to share Btw processes multitasking Best of both thread –one process –multitasking Q: browsers to use multi-process?

CSC 360, Instructor: Kui Wu 3 1. What is thread (2): Threads Thread a basic unit of CPU utilization –thread state, program counter, register set, stack share with other threads in the same process –code, data, opened files, signals, etc Benefits responsiveness: multithreading resource sharing, efficiency, MP architectures Q: potential problems?

CSC 360, Instructor: Kui Wu 4 1. What is thread (3): Single- threaded Web server Web server with cache and disk wait for a request process the request –check cache; if hit, break –otherwise, retrieve from disk (relatively slow) respond the request One request at a time or create a new process on each request –expensive!

CSC 360, Instructor: Kui Wu 5 1. What is thread (4): Multi- threaded Web server Dispatcher thread wait for a request handoff the request Worker threads process the request –disk I/O respond the request “Many” requests at a time

CSC 360, Instructor: Kui Wu 6 2. User vs kernel threads User threads: e.g., pthread library each process schedules its own threads no context switch between these threads a blocking call blocks the entire process Kernel threads: in almost all modern OS kernel manages all threads can pickup another thread if one blocks Hybrid approaches

CSC 360, Instructor: Kui Wu CSc 360Overview 7 3. Thread models (1) User-kernel mapping many-to-one: low cost, (lower) parallelism one-to-one: high parallelism, (higher) cost many-to-many: limited kernel threads

CSC 360, Instructor: Kui Wu 8 3. Thread models (2) Two-level model

CSC 360, Instructor: Kui Wu 9 4. Threading issues When a new process is created fork(), and usually then exec() –duplicate all threads or just the calling thread? When a signal to be delivered signal: event notification to be handled –to all, some, or a specific thread? Thread pool keep a pool of threads to be used –and reuse

CSC 360, Instructor: Kui Wu Pthread library (1) Create a thread –int pthread_create (thread, attributes, start_routine, arguments); PC: start_routine(arguments); default attributes: joinable and non-realtime Exit from a (created) thread –void pthread_exit (return_value); cleanup handlers by pthread_cleanup_push (); –stack-like “reverse” execution order

CSC 360, Instructor: Kui Wu Pthread library (2) Wait a target thread to exit: synchronize –int pthread_join (thread, return_value); release resource allocated to the target thread Put a target thread in detached state –int pthread_detach (thread); no other threads can “join” this one –no “pthread_attach” resource released once the thread exits –thread can be created in detached state

CSC 360, Instructor: Kui Wu Pthread (3) Cancel another thread –int pthread_cancel (thread); calling thread: send a request target thread: pthread_setcancelstate (); –ignore the request –terminate immediately ·asynchronous cancellation –check whether it should be cancelled periodically ·deferred cancellation

CSC 360, Instructor: Kui Wu Example (1): producer-consumer Multi-process shared memory solution message passing solution Single-process, multi-thread #include... void *producer (void *args); void *consumer (void *args); typedef struct {...} queue;

CSC 360, Instructor: Kui Wu 14 queue *queueInit (void); void queueDelete (queue *q); void queueAdd (queue *q, int in); void queueDel (queue *q, int *out); int main () { queue *fifo; pthread_t pro, con; fifo = queueInit (); if (fifo == NULL) { fprintf (stderr, "main: Queue Init failed.\n"); exit (1); } pthread_create (&pro, NULL, producer, fifo); pthread_create (&con, NULL, consumer, fifo); pthread_join (pro, NULL); pthread_join (con, NULL); queueDelete (fifo); return 0; } 6. Example (2): Main thread

CSC 360, Instructor: Kui Wu Example (3): Producer thread void *producer (void *q) { queue *fifo; int i; fifo = (queue *)q; for (i = 0; i < LOOP; i++) { /* produce LOOP items, inserting them into * the “fifo” queue. */... } return (NULL); }

CSC 360, Instructor: Kui Wu Example (4) Consumer thread void *consumer (void *q) { queue *fifo; int i, d; fifo = (queue *)q; for (i = 0; i < LOOP; i++) { /* Consumer LOOP items from the * “fifo” queue. */... } return (NULL); }